1. Field of the Invention
The present invention relates to a method for manufacturing a polymerized toner, and more specifically to an efficient method for manufacturing a polymerized toner having a very sharp particle diameter distribution and an extremely small quantity of fine powder. The manufacturing method of the present invention is particularly suitable as a method for manufacturing a small particle diameter polymerized toner having a volume average particle diameter within the range between 4 and 9 μm.
2. Description of the Related Art
In image forming machines such as electrographic or electrostatic copying machines, laser-beam printers, and facsimiles, a developer is used for visualizing electrostatic latent images formed on a photoconductive member. The developer contains colored particles (toner) containing a colorant, a charge control agent, a release agent and the like dispersed in a binder resin as the major component thereof.
Toners are roughly divided into pulverized toners obtained by a pulverizing method, and polymerized toners obtained by a polymerization method. In the pulverizing method, a thermoplastic resin are melted and kneaded together with additive components such as a colorant, a charge control agent, and a release agent, and the mixture is pulverized and classified to obtain a pulverized toner as colored resin particles. The pulverized toner has an indeterminate form and a broad particle diameter distribution. Moreover, since fine powder (denoting excessively fine particles hereinafter) is easily formed by pulverization in the pulverized toner, it is difficult to manufacture a small-particle-diameter toner having a desired average particle diameter at a high yield.
In the polymerization method, a polymerizable monomer composition containing a polymerizable monomer, a colorant, and other additive components is polymerized in an aqueous dispersion medium to form a polymerized toner as colored polymer particles. In the polymerization method, the formed colored polymer particles are washed, filtered, and dried after polymerization to obtain a polymerized toner.
According to the polymerization method, a spherical polymerized toner with a relatively sharp particle diameter distribution can be manufactured. Also according to the polymerization method, core-shell structure colored polymer particles can be formed by polymerizing a polymerizable monomer for the shell after the polymerization of the polymerizable monomer composition, in the presence of the formed colored polymer particles. If the glass transition temperature of the polymer component constituting the core is lowered, and the glass transition temperature of the polymer component constituting the shell is elevated, a polymerized toner that excels both in shelf stability and low-temperature fixing properties can be manufactured. Furthermore, according to the polymerization method, a polymerized toner having a volume average particle diameter of 10 μm or less, preferably 4 to 9 μm, can be easily manufactured. The polymerized toner, can therefore form a high definition and high quality image and is suitable also for increasing a printing speed and forming a full-color image.
Thus, a polymerized toner of a small particle diameter plays an extremely important role on the formation of high-precision and high-quality images. However, various problems have arisen as the particle diameter of the polymerized toner is reduced. For example, in order to lessen the particle diameter of the polymerized toner, the dispersion containing a polymerizable monomer composition must be dispersed using an agitator, to form fine droplets of the polymerizable monomer composition, in the droplet forming step preceding the polymerizing step. However, if the particle diameter of the droplets becomes less, the viscosity of the dispersion elevates significantly, resulting in the difficulty of forming the droplets (the process for forming fine droplets), and lowering the agitating efficiency of the agitator.
In addition, when the particle diameter of the droplets becomes less, the proportion of formed fine powder increases remarkably, and highly sharp particle diameter distribution becomes difficult to materialize. As a result, fog increases, and the printing density lowers. If the proportion of fine powder is large, the fine powder must be removed from the colored polymer particles by classification, resulting in complicated process and high costs. Therefore, in order to form the images of high definition and high quality, the establishment of the art for efficiently manufacturing a polymerized toner is desired, which toner having an adequately small average particle diameter, a very sharp particle diameter distribution, and an extremely low content of fine powder.
Conventionally, there has been proposed a process, wherein a dispersing machine, having a combination of a rotor and a stator that are concentric comb-teeth rings is used as an agitating apparatus, and is constituted so as to drive the rotor to rotate at a high speed to flow the dispersion from the inside of the rotor to the outside of the stator, and to agitate the dispersion in the gap between the rotor and the stator (Japanese Patent Laid-Open No. 2-32363/1990). In other words, in this prior art, a method for manufacturing a polymerized toner is disclosed, wherein the step for dispersing a monomer composition containing a polymerizable monomer and a colorant into an aqueous dispersion medium to form suspension particles (droplets) is a step for flowing the monomer composition from the inside of the rotor toward the outside of the stator.
However, it is difficult to obtain a polymerized toner, having a small particle diameter, an extremely sharp particle diameter distribution, and a small proportion of fine powder, only by adopting a dispersing machine such as Ebara Milder (manufactured by Ebara Corporation).
Then, there has been proposed a method, wherein the tip speed (peripheral velocity) of an agitating blade (rotor) of an agitator and the internal pressure of the agitating section are controlled so that the relationship thereof satisfies a specific formula, in the process for forming fine droplets of a polymerizable monomer composition using an agitating machine such as Ebara Milder (Japanese Patent Laid-Open No. 2002-221824). According to the method disclosed in this prior art, a polymerized toner having a small particle diameter and a sharp particle diameter distribution can be obtained. However, the proportion of fine powder is not sufficiently reduced only by controlling the relationship between the rotor of the agitating apparatus (dispersing machine) and the internal pressure, and the speed of the formation of droplets having a desired average particle diameter is not sufficiently high. Therefore the process, materializing sufficient reduction of fine powder and high speed formation of droplets having a desired average particle diameter, is desired.